Electroviscous fluids (electro-rheological fluids) whose viscosity changes upon application of a voltage have been known for a long time (Duff, A. W. Physical Review Vol. 4, No. 1 (1896) 23). At the beginning of the study on electroviscous fluids, attention was paid to systems consisting only of a liquid. Therefore, the electroviscous effect obtained therefrom was insufficient. However, the subject of the study shifted to the electroviscous fluids of solid dispersed systems thereafter, and it has become possible to obtain considerable electroviscous effect.
Regarding the viscosity increase effect (ER effect) manifesting mechanism in electroviscous-fluids, Klass, for example, states that particles which form a dispersoid in an electroviscous fluid cause induced polarization of the double layer in an electric field, and the induced polarization accounts for the manifestation of the ER effect (Klass, D. L., et al., J. of Applied Physics, Vol. 38, No. 1 (1967) 67). Let us explain the mechanism on the basis of the electric double layer: Ions which are adsorbed around a dispersoid (silica gel or the like) are uniformly disposed on the outer surface of the dispersoid when E (electric field) is 0. However, when E (electric field) assumes a finite value, the ion distribution is deviated, causing the particles to exert electrostatic action on each other in the electric field. Thus, the particles form a bridge between the electrodes, thus manifesting shear resistance to stress, that is, ER effect.
Winslow proposed an electroviscous fluid using a paraffin, silica gel powder, and water as a polarizing agent (Winslow, W. M, J. of Applied Physics, Vol. 20 (1949) 1137). By virtue of the Winslow's study, the electroviscous effect of electroviscous fluids is called "Winslow effect".
In such an electroviscous fluid, porous solid particles are used as a dispersoid. However, such a dispersoid involves a problem in terms of dispersibility: If the electroviscous fluid is allowed to stand for a long time, a solid precipitate is formed. Under the temperature conditions of about 100.degree. C. the electroviscous fluid forms a gel-like substance on standing for only a short time of from several minutes to several hours, resulting in a failure to function as an electroviscous fluid. To improve the dispersion stability, the conventional practice is to finely divide solid particles dispersed in the electroviscous fluid to the level of the critical particle diameter and to add a dispersant such as polybutenyl succinic acid imide. However, it has been proved that polybutenyl succinic acid imide has a high molecular weight, and since the molecular length of the dispersant is excessively long in comparison to the particle diameter, it is impossible to obtain sufficient attraction force between the particles and hence impossible to obtain the desired electroviscous effect. In terms of thermal setting also, the conventional electroviscous fluids are considered likely to cause aggregation of particles under heating conditions.
Japanese Patent Application Post-Examination Publication No. 45-10048 discloses an electroviscous fluid which is a dispersion of esterified silica particles in an electrically insulating fluid having a high base viscosity. The esterified silica particles have a particle diameter of from 0.04 um to 10 um, and have about 0.5 to 1.5 silica-bonded OR groups per nm.sup.2 of the particle surface, and from 1 to 3 molecules of free water, wherein R is an ester residue of a polyoxy-substituted ester or polyoxyalcohol having a molecular weight of from about 130 to 400. However, silica particles esterified with a polyhydric alcohol are still likely to aggregate, and involve the problems that the degree of esterification is low, and the standing stability is inferior. Further, since water is used as a polarization promoter, the electroviscous effect under high-temperature conditions is unstable. In addition, if silica particles having a relatively large particle diameter are dispersed in an electrically insulating fluid having a low base viscosity, precipitation is likely to occur, giving rise to a problem.
An object of the present invention is to provide an electroviscous fluid which uses a polyhydric alcohol as a polarization promoter in a non-aqueous system, and which is excellent in dispersion stability and shelf stability, free from aggregation of particles even under heating conditions and capable of manifesting high electroviscous effect.